The present invention relates generally to compensation circuits used with telecommunication transmission facilities and, more particularly, to automatic gain and loss compensation circuits used in remote terminal units. Many telecommunications transmission facilities include a central office which may transmit data signals over transmission lines to remote terminals and customer premises. The signals may be sent over two pairs of transmission lines in a digital format or differentially on two conductors, known as the Tip-Ring Pair.
The Bell telephone system in the United States, for example, has a widely utilized Digital "D" multiplexing pulse code modulation systems. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on a T-1 transmission system. One pair of cables is provided for each direction of transmission.
Signals which are sent via digital carrier transmission lines from the central office reach a remote terminal before reaching the customer premises. The remote terminal then converts the digital signals to an analog signal which may then be an intelligible signal for the telephone.
Thus, in the remote terminal, the digital data signals, sent over the T-1 lines, are convened to analog telephone signals and then supplied to a customer loop telephone line. The analog telephone signals may then be received by the customer premises, which may include telephones and, in some cases, PBX units. Conversely, the remote terminals receive analog telephone signals from the telephones and PBX units and convert them to digital data signals, which then can be transmitted over the T-1 transmission system.
The length of the telephone line between the remote terminal and the PBX unit or telephone in the customer premises may vary substantially. The length of the line affects the impedance of the line and, thus, for example, the amplitude of signals between the remote terminal and the customer premises.
If the remote terminal provides an analog telephone signal to the telephone line at a predetermined gain, regardless of the length of the telephone line, the sound on the telephone may be too loud if the telephone line is relatively short and may be too weak if the distance is relatively long. With many presently available remote terminals, the amount of amplification, or gain, that the remote terminal uses in converting the digital data signals to analog telephone signals or analog telephone signals to digital data signals may vary by manually adjusting the gain of the converter to be consistent with the length of the telephone line in that particular location.
SUMMARY OF THE INVENTION
In a principal aspect, the present invention relates to a gain and loss compensation circuit for a digital transmission system. The digital transmission system includes a digital carrier transmission line and a customer loop telephone line. The circuit includes a converter, an interface circuit, a gate array, and a microprocessor. The converter is interconnected to the receive transmission line, accepts the digital data signal, and converts it to an intermediate analog signal. Conversely, the converter also receives an intermediate analog signal from the interface circuit, converts it to a digital data signal, and transmits it along the transmit transmission line. The operation of the converter, and, thus, the gain used by the converter in performing such conversions, is related to a compensation signal that the converter receives.
The interface circuit receives the intermediate analog signal from the converter and provides an analog receive telephone signal to the telephone line, which is ultimately interconnected to the customer's telephone. Conversely, the interface circuit also receives an analog transmit telephone signal and provides an intermediate analog signal to the converter.
The interface circuit also includes a pulse width modulated power converter. The power converter provides pulses having a particular pulse width. Further, the interface circuit provides a direct current along the telephone line, and the amplitude of the current is related to the pulse width.
The gate array senses the pulse width of the power converter in the interface circuit and responsively provides a measured signal to the microprocessor. The microprocessor receives the measured signal and responsively provides compensation signal to the converter, so that gain of the converter is consistent with the length of the telephone line between the remote terminal and customer premises.
Thus, an object of the present invention is an improved gain and loss compensation circuit within a digital transmission line system. Yet another object is an improved gain and loss compensation circuit that more automatically adjusts the gain of a remote terminal unit. These and other objects, features, and advantages of the present invention are discussed or are apparent in the following detailed description.